The research proposed here is directed toward determining which neural systems and related functions are most dependent on and modified by experience (i.e. input from the environment) during human development. We will characterize the time periods (sensitive periods) when particular perceptual and language systems display experience-dependent changes and contrast these systems with those that retain the ability to change throughout life. In these studies we will acquire event-related brain potentials (ERPs) and structural and functional magnetic resonance images (MRI) of the brain to precisely characterize both the timing and the location of neural activations as these groups of subjects perform tasks designed to activate specific aspects of sensory, attentional and language functions. In a series of studies investigating neuroplasticity of visual perception and attention, we will assess the hypotheses that: (1) congenially deaf subjects (Ss) are more sensitive to and display more extensive neural activations than hearing Ss to both attended and unattended visual stimuli, especially in the far periphery (>20[unreadable]) of the visual fields;(2) the increased neural activations in deaf compared to hearing Ss occur in lower- level and higher-level visual areas, multimodal areas and primary and secondary auditory cortices;and (3) certain alterations in white matter tracts in congenially deaf individuals are attributable to auditory deprivation while others are a consequence of reduced experience in reading. In studies investigating neuroplasticity of subsystems of language we will (1) assess and compare indices of temporally-based attention and speech segmentation to test the hypothesis that domain-general aspects of attention are central to speech perception skills in native and late learners of English;(2) characterize the identity and time-course of operation of the neural subsystems important in syntactic and semantic processing in normal hearing monolingual adults who vary in language proficiency;(3) assess and compare the effects of delays in second language (L2) acquisition and level of language proficiency on these same subsystems by studying bilinguals who acquired L2 late and who attained different proficiency levels;and (4) assess the hypotheses that the neural systems active when native signers process phonological information in ASL overlap extensively with systems active when native speakers process English phonology and that these systems are dependent on early exposure to ASL. Since the proposed research will determine aspects of perception, attention and language that are most altered by environmental input and the time periods when they are most modifiable, these studies can contribute information of practical significance for the development, refinement and evaluation of educational programs for hearing and deaf individuals and late learners of a second language.